Black tea contains more of the substance, a polysaccharide compound, than either green or oolong tea, report Haixia Chen and colleagues of Tianjin University, China.

Coarse tea has been used as a diabetes treatment in China and Japan. It's known that tea polysaccharides reduce blood sugar.

Now Chen and colleagues show that tea polysaccharides inhibit an enzyme called alpha-glucosidase, which turns starch into glucose. The diabetes drugs Precose and Glyset work by inhibiting this enzyme.

"Many efforts have been made to search for effective glucose inhibitors from natural materials," Chen says in a news release. "There is a potential for exploitation of black tea polysaccharide in managing diabetes."

It's not clear whether simply drinking black tea would help. Chen's team used chemical extraction methods -- not simple brewing -- to derive the polysaccharides from tea they purchased at local markets.

Chen and colleagues report their findings in the current issue of the Journal of Food Science.